The effect of aging on bone formation in porous hydroxyapatite: biochemical and histological analysis.

The effect of aging on osteoblastic differentiation of marrow stromal stem cells was examined. Porous hydroxyapatite (HA) disks were soaked in cells suspensions of bone marrow cells from young (8 weeks) and old rats (60 weeks) and then implanted subcutaneously in syngeneic young and old rats. The bone marrow/HA composites were harvested 8 weeks later, and the contents of bone Gla protein (BGP) and alkaline phosphatase (ALP) activity in them were determined. Histologically, bone formation could be detected in all the composites in young recipient rats; however, some old bone marrow/HA composites in old recipients did not show bone formation and the bone volume in the young bone marrow/HA composites was greater than in the old bone marrow/HA composites. The ratios of ALP activities of young bone marrow/HA composites to old bone marrow/HA composites in young and old recipients were about five times and four times, respectively. The ratios of BGP contents of young bone marrow/HA to old bone marrow/HA composite in young and old recipients were about nine and eight times, respectively. The results suggest that the decreased bone formation observed in old bone marrow cells was due to a smaller population of stromal cells and/or decreased capacity of differentiation of stromal stem cells into osteogenic cells.